The Stehle et al U.S. patent application Ser. No. 495,099, filed Aug. 5, 1974, discloses a pressurized-water reactor nuclear fuel rod comprising a cladding tube having end caps welded on its ends so that the tube is gas-tightly closed. The tube contains fuel pellets extending from one closed end to adjacent its other closed end and between the fuel pellets and this other closed end, a fission-gas collecting capsule is positioned, this capsule featuring a connection between its inside and the inside of the cladding tube and the fuel, in the form of a flow choke.
The fission gas collects inside of the capsule, the flow choke being adequate at this time because the fission gas is produced slowly by the fuel when the fuel rod is in service. In the event of a loss-of-coolant accident, the coolant pressure on the outside of the cladding tube drops, the flow choke preventing very rapid flow-back of the fission gas into the fuel-containing portion of the cladding tube during the short time interval required for the normal emergency core cooling system to become effective. Without this flow choke feature, the fission gas would immediately flow back into the fuel-containing space of the cladding tube, expand the latter, and possibly result in its rupture or at least weakening by bulging of the cladding tube. Under normal operating conditions, the cladding tube is, of course, prevented from inward collapse under the coolant pressure, by the support provided by the fuel pellets on the inside, the capsule acting itself as an internal brace for the fuel cladding throughout the extent of the capsule.
It is to be understood that the fuel cladding tube is made as thin as possible in the interest of nuclear efficiency. Therefore, under the coolant pressure the tube would collapse if it were not for the fact that it is generally completely filled by the fuel pellets and the capsule, the latter extending from and contacting with the fuel pellets and the end cap at that end of the tube, both ends of the tube having the usual welded-on end caps to provide the fuel rod with gas-tightly closed ends. This kind of fuel rod does not vent the fission gas from its inside to its outside, thus needing the fission gas-collecting chamber provided by the capsule.
Prior art fuel rods not incorporating the fission gas-collecting capsule, have a compression spring between the fuel pellets and one closed end of the fuel cladding tube, this maintaining the fuel pellets under elastic compression. Using the fission gas-collecting capsule disclosed by the mentioned patent application, the fuel pellets must be positioned by appropriate dimensioning of the capsule which is itself longitudinally rigid to compression, thus requiring relative precise dimensioning of the length of the capsule, to provide positive positioning of the fuel pellets.